A light-emitting element having a layer containing a light-emitting substance between a pair of electrodes, which is used as a pixel, a light source, or the like, is provided for a light-emitting device such as a display device or a lighting system. In such a light-emitting device, reliability of a light-emitting element is related closely with performance of a light-emitting device. For example, when a short circuit is generated between the pair of electrodes of a light-emitting element, a display image is distorted or light with adequate intensity cannot be lighted.
Therefore, development of a light-emitting element capable of emitting light with long-term stability and with few defective elements has recently been promoted. For example, Reference 1: Japanese Patent Application Laid Open No. H9-63771 discloses a manufacturing technique of a light-emitting element that operates with a low driving voltage by using metal oxide having a high work function such as molybdenum oxide. In addition, life-extension effect is also obtained.
However, molybdenum oxide is crystallized easily and the technique described in Reference 1 cannot adequately reduce malfunction of a light-emitting element due to crystallization. In addition, malfunction is also caused due to the generation of a short circuit as well as a life expectancy of a light-emitting element; therefore, counter measures against these causes are necessary.